Recent years have seen tremendous growth in the number and variety of display devices available to the public. Computers (whether desktop, laptop, or notebook), personal digital assistants (PDAs), mobile phones, MP3 music players, and thin LCD TVs are but a few examples. Although some of these devices can use ordinary ambient light to view the display, most include a backlight to make the display visible.
Many such backlights fall into the categories of “edge-lit” or “direct-lit”. These categories differ in the placement of the light sources relative to the output area or face of the backlight, where the output face defines the viewable area of the display device. In edge-lit backlights, a light source is disposed along an outer border of the backlight construction, outside the area or zone corresponding to the output face. The light source typically emits light into a light guide, which has length and width dimensions on the order of the output face and from which light is extracted to illuminate the output face. In direct-lit backlights, an array of light sources is disposed directly behind the output face, and a diffuser is placed in front of the light sources to provide a more uniform light output. Some direct-lit backlights also incorporate an edge-mounted light, and are thus capable of both direct-lit and edge-lit operation.
One example of a direct-lit display is shown in FIG. 1, which illustrates in a schematic sectional exploded view selected optical components of a liquid crystal display television (LCD TV) 10, commercially available from Samsung Corp. as model LTN226W. This figure is not to scale, and some of the features are exaggerated for ease of illustration. The LCD TV 10 has a 22-inch screen size and a 16:9 aspect ratio, meaning the screen is about 10.8 inches (275 mm) in height and 19.2 inches (485 mm) in width. In this regard, height, width, and depth correspond to the Cartesian x-, y-, and z-axes respectively shown in the figure. The LCD TV 10 has an LCD panel 12 viewable by the observer 13, and a collection of components forming a backlight 14 behind the LCD panel 12.
In the backlight 14, light is generated by an array of eight CCFL sources 16, each of which spans the width of the TV screen and which have a uniform center-to-center spacing S of about 33.8 mm. The sources have a circular cross-sectional shape about 3 mm in diameter (diameter=2 R). They are disposed between a patterned diffuser plate 18 and a back reflector 20. The diffuser plate 18 transmits light but with a substantial amount of scattering to provide light diffusing characteristics. The diffuser plate also has on its back surface a printed dot pattern, which pattern is in registration with the light sources 16. That is, the printed dot pattern is more dense at locations directly above the light sources 16 and less dense at locations between neighboring light sources. Thus, the diffuser plate 18 is highly non-uniform over its useful area. The back reflector 20 is an opaque white film or paint coated onto a stiff metal substrate. The setback distance D from the diffuser plate 18 to the sources 16, measured from the back surface of the diffuser plate to the center of the sources, is about 11.1 mm, and the distance T from the sources 16 to the back reflector 20 (measured from the center of the sources to the front surface of the back reflector) is about 5.6 mm. The overall gap dimension G from the diffuser plate 18 to the back reflector, which is measured from the back surface of the diffuser plate to the front surface of the back reflector, and which satisfies G=D+T, is about 16.7 mm. Atop the diffuser plate 18 is a film stack consisting of: a beaded diffuser film 22 with an outer beaded layer facing the front of the backlight/display as shown; a prismatic brightness enhancement film 24 (Vikuiti™ Brightness Enhancement Film-III-Transparent (BEFIII-T), available from 3M Company) oriented as shown, with prisms facing the front of the backlight/display; and a reflective polarizer 26 (Vikuiti™ Dual Brightness Enhancement Film-Diffuse 440 (DBEF-D440), available from 3M Company). The reflective polarizer 26 can be considered to form the outermost film or layer of the backlight 14.
The backlight 14 illuminates LCD panel 12, which comprises an electronically addressable liquid crystal array sandwiched between glass plates. The panel 12 also includes a front and back absorbing polarizer, and a color filter matrix.
Rather than requiring a spatially non-uniform diffuser plate that must be in registration with the light sources in order to achieve the required illumination uniformity over the useful area of the backlight, it would be desirable to use an alternative technique that does not require spatial nonuniformities to provide the bulb-hiding function.